The present invention relates to a process for the continuous casting of thin metal products and to an apparatus for carrying out the same, and, more in particular to an improved continuous casting method and to an apparatus suitable for carrying out such method, of the kind constituted of two counter-rotating rolls.
Apparatus with counter-rotating rolls for performing continuous casting of metal strips are already known in the state of the art. By means of these apparatus it is possible to jump from the conventional technologyxe2x80x94for obtaining flat products with a thickness ranging from 150 mm to 250 mm, that generally are subjected to a further hot rolling and possibly a subsequent cold rolling, to the so-called xe2x80x9cstrip castingxe2x80x9d technology. Such technology allows to obtain flat or strip products having a thickness lower than 10 mm, that subsequently can be hot/cold rolled.
Recently, the abovesaid technologies have been utilised for the production of steel strips. In particular, according to the more widespread xe2x80x9cStrip Castingxe2x80x9d technology i.e. the so-called xe2x80x9cTwin Rolls,xe2x80x9d technology, there is provided for the casting of molten metal, such as steel, in a molten bath defined by a pair of counter-rotating rolls, which are cooled at their interior, and which have their longitudinal axes parallel and arranged horizontally, spaced each-other by a distance substantially corresponding to that of the cast strip, and a pair of containment side walls or dams made of a refractory material, that are placed in contact with the end surfaces of said rolls.
The sliding contact between the side walls or dams and the rolls must guarantee the absence of leaks of molten metal between them. This entails that said side walls or dams and the end surfaces of the rolls are subjected to particularly severe operating conditions.
Appreciable thermal deformations that modify the initial geometry of the side walls or dams are caused by a pre-heating that is carried out at two moments: i.e. before their abutting against the end faces of the rolls and after their abutting during the start operations before the actual steady-state casting.
The need of such pre-heating derives from the need of reducing to a minimum the likelihood that on the side walls or dams a undesired solidification of steel might occur, that if caught between the rolls, would cause unacceptable defects on the strip and damages to the side walls themselves.
Moreover, also the rolls are subject to thermal deformation during the casting, both in their radial and axial directions, which modify the shape of their end faces which contact the side walls,
It derives therefrom that at room temperature the two surfaces, i.e. the ones of the side walls and the ones Of the end faces of the rolls, lie in a same plane, whereas such condition is lost during the initial transient and during the casting, This entails the build-up of gaps along the coupling areas of the two surfaces with consequent leaks of metal.
A leak or spilling of metal produces consequently detects along the edges of the strip products and unevenness in carrying out the process that can lead, in the most severe cases, to a stop of the plant.
The mating surfaces of the side walls and of the end faces of the rolls are moreover subject to wear owing to their relative motion and to the abutment pressure.
Such wear reduces the useful life of the components, and increases he costs; and it is greater with the increase of the contact forces applied for maintaining the mating of said surfaces.
To the end of overcoming such inconveniences, various contrivances have been developed. As an example in the Published European Patent Applications No 546 206 and 698 433, there is disclosed that the side walls are brought in abutment with the end faces of the rolls as a rigid body, and each side wall is supported on its back by a metal plate whereon pushing forces are applied, forces that then are distributed on the whole surface of the side wall. The side wall or dam pushed in this way against the end faces of the rolls, is subjected to a wear of the parts contacting the end faces of the rolls and therefore to a forward shift of the whole side wall up to when the whole sliding surface of the side wall or dam has reached the same contour of the opposing surface of the end face of the roll, trying in this way to guarantee a seal against penetration and leaks of molten steel.
These methods are however subject to the following inconveniences:
a)the pushing force is initially localised only on the contact areas between the side wall and the surface of the end faces of the rolls, with local values of the pressure that cannot be determined in advance and that can be very high, leading possibly the fracture of the refractory material as well as a relevant wear of the same;
b)a certain time is needed so that the surface of the side wall wears and consequently matches the surface of the end face of the roll (for instance in the European Patent Application No 546 206 above referred to, a time ranging from 0.5 to 1 minutes is mentioned before starting the casting operations).
Consequently the side walls are brought in abutment with the end faces of the rolls after the pre-heating step, with a consequent decrease of the temperature of the side wall or dam because the end surfaces of the rolls are usually cooled. It derives therefrom that such step must be as short as it is possible in order to avoid an excessive cooling of the side wall, because this would lead to solidification of the molten steel on the surface of the same;
c)during the initial steps of the casting, the contact of the molten steel with the rolls and with the side walls leads to a very fast deformation of these bodies and of the abutting surfaces. Such deformation impairs the matching between the surface of the end face of the rolls and the surface of the side walls. It is therefore necessary to wait again the wear of the surfaces of the side walls up to reach a new contour that allows the matching thereof;
d)the use of refractory side walls supported in their back by a metal plate renders very difficult the heating of the side walls from a surface opposite to the one that contacts the molten steel. It derives therefrom that these side walls must be heated from the interior of the space destined to contain the molten steel by means of a device that must be retracted before the starting of the casting (see European Patent Application No 698 433 A1). During the time span between the retraction of the heating device and the start of the casting of the steel, the side walls are subject to cooling. Moreover no mention is made of the possibility of heating the side wall (s) during the casting process. This increases risks of undesired solidification of steel on the side walls and a possible damage of the same.
Consequently, notwithstanding the improvements brought forth in this field, the problems of the confinement of the molten metal in the casting cavity, and of the undue wear of the side walls, and of the quality of the edges of the cast strip, and indeed the stopping of the continuous casting process, are largely still unresolved.
The object of the present invention is therefore to reduce to a minimum the effects caused by the abovesaid problems by providing a method and apparatus for the continuous casting of thin material, improved according to the method disclosed hereinafter, in order to reduce the possibility of leaks of molten metal between the surface of the side walls or dams and the surfaces of the end face of the rolls, and at the same time to reduce the wear both of the surface of the side walls and the surface of the end faces of the rolls.
Another object of the present invention is to reduce greatly the amount of defects in correspondence with the edges of the cast product.
A still further object of the present invention is to reduce to a minimum the likelihood of stops in the continuous casting process, caused by leaks between the side walls and the end face of the rolls, as well as to reduce to a minimum the likelihood of the solidification of steel on the side walls.
According to the present invention, there is provided a method for the continuous casting of thin metal products, comprising the operations of pre-heating at least a pair of refractory plates for the lateral containment of a bath of molten metal constituted of a pair of counter-rotating rolls and arranged parallel each other and spaced for an amount greater than the sum of their radiuses and substantially corresponding to the thickness of the metal product, and of abutment of said at least a pair of plates towards each lateral surface of the ends of said pair of rolls, the method is characterised in that it comprises the following operations:
to apply a controlled deformation to said at least a pair of plates in correspondence with the contact arc between the surface of the plates and the peripheral portion of the ends of said rolls by means of thrust means contacting said plates, on the basis of data collected in advance and processed by means of a mathematical model representative of the behaviour of plates and rolls, with the same chemical-physical and dimensional characteristics of those utilised in the same experimental conditions; and
controlling said deformation of said at least a pair of plates during the whole casting process by means of said thrust means and a control unit connected therewith, in order to mantain the surface wear of the plates and of the ends of the rolls within a minimum value, to maintain a distance between said rolls and said plates lower than a predetermined value, and to render minimum the occurrence of leaks of molten metal between said plates and said end parts of said rolls.
Further, the present invention provides an An apparatus for the improved continuous casting of thin metal products, comprising a pair of counter-rotating rolls arranged parallel each other at a distance greater than the sum of their radiuses and substantially corresponding to the thickness of said metal product, and a side containment device arranged on each lateral surface of each end of said pair of rolls, wherein said lateral containment device comprises a frame made of metal and a plate made of refractory material housed within said frame, and thrust means closely coupled to said frame on the back of the plate of refractory material and in register with the contact arc between said plate and the end faces of said rolls;
characterised in that said side containment device further includes:
heating means for said plate of refractory material rigidly arranged on said frame, in order to direct the heat onto the back surface of the plate;
means for detecting the deformation of said plates and associated to said thrust means; and in that it comprises a control unit having its input connected to a data entry unit and on its output connected to said pair of rolls and said side containment device.
In the apparatus according to the present invention, said thrust means comprises:
a plurality of ceramic cylinders;
a plurality of oil-pressure actuators fastened on said frame and coaxially with said ceramic cylinder; the arrangement being such so that for each and any displacement of said actuators there corresponds a displacement of the corresponding ceramic cylinder.
Further, in the apparatus according to the present invention, said heating means comprises at least a burner arranged on said frame, and so that it directs its flame onto the back surface of the plate of refractory material.
Further, in the apparatus of the present invention, said means for detecting the deformation of said plates are associated to said oil-pressure actuators, and are position transducers.